#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n';
const int M = 4e5+5, MOD = 1e9+7;
int dp[M][2][2], x[M];
vector<int> node[M];
void dfs(int s, int p = 1) {
for (int i:node[s]) {
if (i != p) dfs(i, s);
}
int cnt = 0, a = 0;
for (int i:node[s]) {
if (i != p) {
cnt += dp[i][0][1] < dp[i][0][0];
a += min(dp[i][0][0], dp[i][0][1]);
}
}
dp[s][0][0] = dp[s][1][0] = INT_MAX;
for (int i:node[s]) {
if (i != p) {
if (x[s] != cnt%2) {
dp[s][0][0] = min(dp[s][0][0], a+abs(dp[i][0][0]-dp[i][0][1]));
dp[s][1][0] = a;
} else {
dp[s][1][0] = min(dp[s][1][0], a+abs(dp[i][0][0]-dp[i][0][1]));
dp[s][0][0] = a;
}
}
}
cnt = 0, a = 0;
for (int i:node[s]) {
if (i != p) {
cnt += dp[i][1][0] < dp[i][1][1];
a += min(dp[i][1][0], dp[i][1][1]);
}
}
dp[s][0][1] = dp[s][1][1] = INT_MAX;
for (int i:node[s]) {
if (i != p) {
if (x[s] != cnt%2) {
dp[s][0][1] = min(dp[s][0][1], a+abs(dp[i][1][0]-dp[i][1][1]));
dp[s][1][1] = a;
} else {
dp[s][1][1] = min(dp[s][1][1], a+abs(dp[i][1][0]-dp[i][1][1]));
dp[s][0][1] = a;
}
}
} dp[s][0][1]++, dp[s][1][1]++;
if (node[s].size() == 1 && s != 1) {
dp[s][0][0] = x[s] *INT_MAX;
dp[s][1][0] = (!x[s])*INT_MAX;
dp[s][1][1] = x[s] *INT_MAX+1ll;
dp[s][0][1] = (!x[s])*INT_MAX+1ll;
}
for (auto&z:dp[s]) for(auto&w:z) w = min(w, (int)INT_MAX);
}
signed main() {
cin.tie(0)->sync_with_stdio(0);
int n;
cin >> n;
for (int i = 1; i < n; i++) {
int a, b;
cin >> a >> b;
node[a].push_back(b);
node[b].push_back(a);
}
for (int i = 1; i <= n; i++) cin >> x[i];
dfs(1);
// for (int i = 1; i <= n; i++) {cout << i << ": ";
// for (auto&z:dp[i]) for(auto&w:z) cout << w << ' '; cout << endl;
// }
int ans = min(dp[1][0][0], dp[1][0][1]+1);
cout << (ans<INT_MAX?to_string(ans):"impossible") << endl;
return 0;
}
/*
4
10 4 49 22
5
1 2
1 3
2 4
2 5
0 1 0 1 1
5
1 2
2 3
3 4
4 5
0 1 1 1 1
impossible
*/
